I playing 3 days, today all my progress deleted. WHY!? I play demo on smart phone because I haven't PC, I want buy this game. Can I save a progress on my phone????
Most likely your mobile browser is configured to erase cookies and/or cache automatically every few days, and the game progress was saved in one of those two ways (I'm not affirming this, only hypothesizing it). Check your browser's privacy settings.
Don't buy the game on mobile yet by the way, it has been abandoned by the developers because it was written in an outdated game engine. They will most likely re-launch the mobile version in the future. In the mean time you can play Magic Survival for Android, which was the inspiration for Vampire Survivors (the VS devs even say so in one of their update posts on Steam). Magic Survival will be ported to iOS soon as well.